The notice said some meat produced in China and Mexico was contaminated with clenbuterol, a muscle-building and metabolism increasing stimulant that sports leagues have banned under the category of performance-enhancing drugs.

The drug is not just used to amp up the athletic prowess of players, but is also injected into livestock to make them larger — a practice strictly forbidden in Mexico and China but still flourishing.

“Consuming large quantities of meat while visiting those particular countries may result in a positive test for clenbuterol in violation of the policy,” NFL said.

The message has dismayed players who are now forced to go on a vegetarian diet while in these countries.

But one player already knows the risks of eating meat in Mexico. Duane Brown, who plays for the Houston Texans, spent a week eating beef in the country, after which he tested positive for clenbuterol. Only by providing detailed description of what he had eaten did Brown avoid suspension.

Some players were not so lucky. Spanish cyclist Alberto Contador was stripped of his 2010 Tour de France title and hit with a two-year ban for having clenbuterol in his system despite claiming it was a result of eating bad meat.

The Mexican soccer team itself tested positive for the banned drug and narrowly escaped charges of doping but only after it missed out on a major tournament.

Even though the PED is banned for use in livestock, last year Mexican authorities suspended 58 out of the 200 slaughterhouses inspected for injecting their cattle with clenbuterol.

In 2011, China, the largest pork exporter in the world, conducted a crackdown and arrested 989 people involved in the manufacture and selling of clenbuterol.

Although clenbuterol has positive uses for people with chronic breathing disorders, its habitual use can cause permanent cardiopulmonary diseases in humans.